The Entity-Relationship Model:
The Entity-Relationship (E-R) model was proposed by Peter Pin-Shan Chen in 1976. The model was proposed as a basis for unifying the different views of data which include the network and the relational views. The E-R model was based on the perception of a real world that consists of a collection of basic objects called entities and their interrelationships (association). An entity may be a person, a book, or something abstract such as a loan or a holiday.
The E-R model, in general, is used in the field of computing and in particular, for organizing data within databases or information systems. The E-R model consists of a component called the E-R diagram which is used for making a graphical or visual representation of the entities and their interrelationships.
There are also special entity types like associative entities and subtype entities. Associative entities or intersection entities are used for associating or relating two or more entities to form a many-to-many relationship. Subtype entities represent a subset of the occurrences or instances of the parent entity, which is called a super type or a generic entity. Subtype entities have attributes or relationships that apply only to the subset.
A non-identifying relationship occurs when the primary key of the child entity does not contain all the attributes of a foreign key. In this relationship, both the parent and the child entities are independent, i.e., the child entity can be identified without the parent.
Attributes can be classified as identifiers or descriptors. Identifiers or keys are special attributes which are used for identifying the unique characteristics of a specific entity. For example, the employee identification number is an identifier that uniquely identifies the employee. Descriptors, on the other hand, identify the non-unique characteristics of an entity.
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